Optical interconnect for high speed processor

ABSTRACT

Data is transferred between a plurality of processors. A plurality of light beams is produced. The light beams have at a particular time a single wave length. The single wavelength is selected from a plurality of wavelengths. Each light beam is uniquely associated with a processor. Each processor selectively modulates data onto its light beam. At a particular time, the selected wavelength of the single wavelength and the processors modulating data onto their light beam is controlled. The plurality of light beams is received at an input ring array. The light beams from the input ring array are transferred to an output ring array. Each light beam is rotated. Each selected wavelength is uniquely associated with a particular rotation. A light beam is output to each processor from the output ring array.

This application is a continuation of U.S. patent application Ser. No.09/847,180, filed May 2, 2001 now U.S. Pat. No. 6,373,605, which is acontinuation of U.S. patent application Ser. No. 09/430,543, filed Oct.29, 1999 now U.S. Pat. No. 6,243,180, which is a continuation of U.S.patent application Ser. No. 08/896,367, filed Jul. 18, 1997, now U.S.Pat. No. 6,008,918, which is a continuation of U.S. patent applicationSer. No. 08/641,632, filed May 2, 1996, now U.S. Pat. No. 5,677,778.

BACKGROUND

This invention relates to an optical free space interconnect forultra-high speed single instruction multiple data processors.

As the geometries of VLSI grow smaller and denser electronicinterconnects and heat dissipation have been recognized as bottlenecksof advanced electronic computing systems. F. E. Kiamilev and et al.,PERFORMANCE COMPARISON BETWEEN OPTOELECTRONIC AND VLSI MULTISTAGEINTERCONNECTION NETWORKS, J. Lightwave Tech., vol.9, pp. 1674-1692,1991. M. R. Feldman, S. C. Esener, C. C. Guest, and S. H. Lee,COMPARISON BETWEEN OPTICAL AND ELECTRIC INTERCONNECTS BASED ON POWER ANDSPEED CONSIDERATIONS, Appl, Opt., vol. 27, pp. 1742-1751, 1988.

Furthermore, as systems are operated at higher and higher speeds, thelatency induced by electronic connections becomes a limiting factor.Although some new techniques, such as three dimensional multi-chipmodules, have been developed to provide short connection distances andless latency, the basic limitation of the pin-out problem in electronicconnections cannot be fully removed. L. D. Hutchson and P. Haugen,OPTICAL INTERCONNECTS REPLACE HARDWIRE, IEEE Spectrum., pp. 30-35, 1987.

Optical interconnections, because of their three-dimensional (3-D)processing capabilities and matched impedance characteristic, have beenconsidered as the best alternative to electronic interconnections.Optical implementations of chip-to-chip and backplane-to-backplaneinterconnections have been reported. See, e.g. J. W. Goodman, OPTICALINTERCONNECTIONS FOR VLSI SYSTEMS, Proc. IEEE, vol. 72, p. 850, 1984.Optical 3-D multi-stage interconnection networks have been investigatedand realized. A. A. Sawchuk Proc. SPIE, vol. 813, p. 547, 1987.Recently, a new free space optical interconnect based on ring topologieswas proposed by Y. Li, B. Ha, T. W. Wang, A. Katz, X. J. Lu, and E.Kanterakis Appl, Opt., vol. 31, p. 5548, 1992.

Arranging the input array on a ring, this novel architecture is capableof interconnecting many processors with identical latency and minimalcomplexity, and cost. This architecture is best suitable for theimplementation of Single Instruction Multiple Data (SIMD) streammachines. Most topologies developed for rectangular array, such asNearest Neighbor (NN), Plus Minus 2I (PM2I), and Hypercube can beimplemented using this ring is topology. A simplified architecture ofthe ring topology architecture is depicted in FIG. 1. The opticalinterconnect has an input ring array 42 and an output ring array 45. Theoptical interconnect uses a first plurality of beamsplitters 23, 24, 25,26, connected to the input ring array 42. The first plurality ofbeamsplitters 23, 24, 25, 26 is connected through a plurality of opticalswitches 27, 28, 29, 30, a plurality of dove prisms 56, 57, 58, 59 to asecond plurality of beamsplitters 61, 62, 63, 64. A multiple channelsystem is used to perform a certain interconnection, i.e. a set ofpermutations. The number of channels depends on the topology to berealized. For example, to realize the NN type of interconnect shown inFIG. 2, a four channel system is needed.

SUMMARY

Data is transferred between a plurality of processors. A plurality oflight beams is produced. The light beams have at a particular time asingle wave length. The single wavelength is selected from a pluralityof wavelengths. Each light beam is uniquely associated with a processor.Each processor selectively modulates data onto its light beam. At aparticular time, the selected wavelength of the single wavelength andthe processors modulating data onto their light beam is controlled. Theplurality of light beams is received at an input ring array. The lightbeams from the input ring array are transferred to an output ring array.Each light beam is rotated. Each selected wavelength is uniquelyassociated with a particular rotation. A light beam is output to eachprocessor from the output ring array.

BRIEF DESCRIPTION OF THE DRAWING(S)

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate preferred embodiments of theinvention, and together with the description serve to explain theprinciples of the invention.

FIG. 1 shows general structure of the originally proposed optical ringtopology NN interconnect architecture;

FIG. 2 shows grid array of an NN interconnect architecture;

FIG. 3 shows a wavelength selection, external modulation system;

FIG. 4 shows the structure for an optical ring topology NN interconnectarchitecture;

FIG. 5 shows electronically controlled tunable laser;

FIG. 6 shows general architecture of an interferometric Mach-Zehnderswitch;

FIG. 7 shows a free space power combiner;

FIG. 8a shows an alternate free space power splitter;

FIG. 8b shows a cross section at focal plane;

FIG. 9 illustrates data modulation and coupling to input ring; and

FIG. 10 shows spectral selectivity of a reflection holographic grating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Reference now is made in detail to the present preferred embodiments ofthe invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals indicate like elementsthroughout the several views.

The present invention provides a novel ring topology based architecture.The major features of this architecture are:

1. Instead of direct modulation of the laser diode current, externalmodulation may be used. Under this approach, integrated single elementelectro-optic switches capable of operating in excess of 40 GHz may beused.

2. Multiple wavelength laser diode sources combined with externalswitches are used to control the routing of light to different channels.This eliminates the need of two dimensional (2-D) array switchingdevices. The system performance may no longer be limited by thesedevices.

3. Wavelength selective holographic optical elements(holographic-optical elements) may be used to form the routing functionof the system based on the selected light wavelength.

Broadly, the present invention uses a modulation system connected to anoptical interconnect. The modulation system is external to the opticalinterconnect. The modulation system provides wavelength selection, andmodulates the selected wavelength with data. Broadly, the modulationsystem includes second generating means, selecting means, combiningmeans, dividing means, modulating means, and forming means. Theselecting means is coupled to the first generating means and to thesecond generating means. The combining means is coupled to the selectingmeans. The dividing means is coupled to the combining is coupled to theselecting means. The dividing means is coupled to the combining means.The modulating means is coupled to the plurality of SIMD processors, andto the dividing means. The forming means is coupled to the modulatingmeans.

The second generating means generates a plurality of coherent-lightbeams at a plurality of wavelengths. In response to a control signalfrom the first generating means, the first selecting means selects asingle coherent-light beam at a single wavelength from the secondgenerating means. The combining means combines output light from theselecting means. The dividing means divides the output light into aplurality of equal-length paths. Each equal-length path corresponds toeach of the plurality of SIMD processors. Using data from theappropriate SIMD processors, the modulating means modulates the outputlight. The forming means forms an input ring at an input plane of theoptical interconnect.

In the exemplary arrangement shown in FIG. 3, an optical interconnectfor a plurality of single-instruction-multiple-data (SIMD) processors isprovided using the first generating means. The first generating means isembodied as a controller 21 or equivalently a central processing unit(CPU). The CPU may employ a computer using, by way of example, 8086,80286, 80386, 80486, or Pentium microprocessor, or equivalently a 68040microprocessor, a larger computer system, or an application specificintegrated circuit (ASIC) designed for the optical interconnect.

The second generating means is embodied as a plurality of laser diodes31, 32, 33, 34, for generating a plurality of coherent light beams at aplurality of wavelengths, respectively. Each of the plurality of laserdiodes 31, 32, 33, 34, operates at a different wavelength.

The selecting means is embodied as a first plurality ofoptical-switching devices 36, 37, 38, 39. The first plurality ofoptical-switching devices 36, 37, 38, 39 are coupled to a plurality oflaser diodes 31, 32, 33, 34, respectively. The optical-switching devicescontrol the on/off output of coherent-light beam from their respectivelaser diodes. Thus, a first optical switching device 36 controls whetheror not the light from laser diode 31 is radiated. Similarly, secondoptical switching device 37 controls whether or not the light from laserdiode 32 is radiated. Generally, the plurality of laser diodes 31, 32,33, 34 radiate light continuously. The first plurality ofoptical-switching devices 36, 37, 38, 39 is coupled to the controller 21through control bus 22. The controller 21 controls the operation of eachof the first plurality of optical-switching devices 36, 37, 38, 39.

The combining means is embodied as a fiber combiner and splitter 40. Thefiber combiner and splitter 40 is connected to the plurality ofoptical-switching devices 36, 37, 38, 39. The fiber combiner andsplitter 40 combines output light from the first plurality ofoptical-switching devices 36, 37, 38, 39. An optical fiber, included inthe fiber combiner and splitter 40, is connected between the fibercombiner and the fiber splitter for carrying the output light from thefiber combiner to the fiber splitter. The fiber splitter, which isconnected at the output of the optical fiber, divides the output lightinto a plurality of equal-length paths. Each equal-length pathcorresponds to one of the plurality of SIMD processors 25. Thus, theoutput light from the optical-switching devices is fed to a single fiberwith the use of a fiber combiner. The optical-switching devices arecontrolled by a controller 21 so that at each clock cycle, only onewavelength is selected. Therefore, only one light wavelength is presentat the output of the fiber combiner. The fiber output is divided by afiber splitter into a number of N equal length paths corresponding tothe number of processors in the system. Each path serves one processor.

The modulating means is embodied as a second plurality ofoptical-switching devices 49. The second plurality of optical-switchingdevices 49 is coupled to the output of the fiber combiner and splitter40, and to the plurality of SIMD processors 25 through data bus 26. Thesecond plurality of optical-switching devices 49 modulates the outputlight from the fiber combiner and splitter 40.

Forming means may be embodied as a plurality of optical fibers. Theplurality of optical fibers is connected to the second plurality ofoptical-switching devices 49. The plurality of optical fibers forms aninput ring 42 at an input plane of the optical interconnect. Also shownon FIG. 3, is the output of the optical interconnect which is anoutput-ring array of optical detectors connected to the SIMD processors25 through data bus 27. The plurality of SIMD processors 25 arecontrolled by controller 21 through control bus 23.

Thus, data modulation is performed by placing the second plurality ofoptical-switching devices 49 onto each output of the fiber splitter. Theoutput fibers from the second plurality of optical-switching devices 49are used to form the input ring at the input plane of the free spaceoptical interconnect. The function of the optical system is to route thedata through the proper channel according to the selected lightwavelength. The output data from the interconnect system is then fedback to the Processing Elements (PEs), referred to herein as the SIMDprocessors, of the digital system.

As illustratively shown in FIG. 4, the structure for the optical ringtopology interconnect architecture is depicted. The input system isconnected to the input ring 42, and output detectors, such asphotodiodes, form an output ring array 43. The optical interconnectincludes first reflecting means, image-rotating means, and secondreflecting means. The first reflecting means is depicted as a pluralityof holographic-optical element reflectors 51, 52, 53, 54. Theimage-rotating means is depicted as a plurality of dove prisms 56, 57,58, 59. The second reflecting means is shown as a plurality ofbeamsplitters 61, 62, 63, 64. The initial beam splitter 61 need only bea mirror. The plurality of dove prisms 56, 57, 58, 59 are connectedbetween the plurality of holographic-optical elements 51, 52, 53, 54 andthe plurality of beamsplitters 61, 62, 63, 64, respectively.Accordingly, a first dove prism 56 is connected between a firstholographic element 51 and either a first beam splitter 61 orequivalently a mirror. The second dove prism 57 is connected between asecond holographic-optical element 52 and the second beam splitter 62.The third dove prism 58 is connected between the thirdholographic-optical element 53 and the third beam splitter 63. Eachholographic-optical element reflects light at-one wavelength and istransparent to light at other wavelengths. Light passing through each ofthe dove prisms is at a wavelength according to the holographic-opticalelement corresponding to that dove prism. The beamsplitters 61, 62, 63,64 reflect light at all wavelengths passing through the dove prisms andtheir respective holographic-optical elements.

The plurality of dove prisms 56, 57, 58, 59 are fixed at a specificorientation, i.e. rotation, with respect to the input ring. Accordingly,a dove prism performs a selected interconnection between SIMD processorsaccording to the orientation of the dove prism, rotation about theoptical axis of the dove prism, i.e., with respect to the input ring.The output optical ring is connected to the plurality of SIMD processors25.

In use, the plurality of laser diodes 31, 32, 33, 34 generate aplurality of coherent-light beams at a plurality of wavelengths. Each ofthe coherent-light beams is at a different wavelength from the othercoherent-light beams of the plurality of coherent-light beams. The firstplurality of optical-switching devices 36, 37, 38, 39 are controlled bya control signal from the controller 21. The control signal determineswhich of the optical-switching devices allows light to pass from theplurality of laser diodes 31, 32, 33, 34. Accordingly, by theappropriate control signal, the first plurality of optical-switchingdevices selects a single coherent-light beam from the plurality ofcoherent-light beams, at a single wavelength. The fiber combiner andsplitter 40 combines output light from the first plurality ofoptical-switching devices 36, 37, 38, 39, and divides the output lightinto a plurality of equal-length paths. Each of the equal-length pathcorresponds to each of the plurality of SIMD processors.

Data from the SIMD processors, at a given moment in time, modulates theoutput lights from the fiber combiner and splitter 40. The controller 21controls which of the SIMD processors 25 modulates the output light. Theplurality of optical fibers forms an input ring at the input plane ofthe optical interconnect, so that output light at any wavelength has thesame path in length into the optical interconnect.

In the optical interconnect, each of the plurality ofholographic-optical elements 51, 52, 53, 54 reflects light at a singlewavelength and is transparent to light at other wavelengths. Therefore,only one optical channel is operational for each wavelength. Selectedpermutations are accomplished by controlling the optical-switchingdevices 36, 37, 38, 39. Clearly only one of them is on at a time.Absorption and diffraction efficiency of the holographic-opticalelements are the two factors which may induce power loss in the freespace optical interconnect system. Absorption in the infrared region isvery low for photopolymer holographic materials. A diffractionefficiency of more than 99% has been obtained for a reflection typehologram. Therefore, the free space optical system may attain nearly100% light efficiency. The power loss due to having all lasers operatingat all times may be alleviated by using a single tunable laser diode 66,see FIG. 5. However, this puts restrictions on the switching speedbetween wavelengths and the selectivity of the holographic-opticalelements.

The plurality of dove prisms 56, 57, 58, 59 each perform at a giventime, a selected fixed interconnection between SIMD processors accordingto the orientation of the dove prism, rotation about the optical axis ofthe dove prism, i.e., with respect to the input ring. The opticalchannel is determined by the control signal from the controller 21 whichcontrols the first plurality optical-switching devices 36, 37, 38, 39.The plurality of beamsplitters 61, 62, 63, 64 are coupled to theplurality of dove prisms 56, 57, 58, 59 and reflect the light from theselected optical channel to the output-ring detector array 43. Theoutput ring detector array 43 detects the output light from the selectedoptical channel and the data are fed on the data bus 27 to the SIMDprocessors 25.

The channel selecting means alternatively may be embodied as internalmodulators to the lasers. The light admitted by the lasers thus ismodulated by altering the electric current which drives each laser.While FIG. 3 shows external light modulation, internal modulation mayalso be used depending on the processor speed and may be sufficient.

FIG. 7 illustratively shows the light admitted by all lasers may becombined and then split approximately at equal powers to a set ofcomponents the number of which equals the number of processing elements.There are many alternatives in performing this function.

FIG. 3 illustrates an efficient method when a small number of processorsare involved. For a large number of processors, other more efficientmethods as the one illustrated in FIGS. 7 and 8 may be employed.

FIG. 8a shows an alternative free space power splitting system whichincludes phase grating 86 and optical system 87. A cross section atfocal plane 88 is shown in FIG. 8b as a grid array of line spectra.

FIG. 9 shows the data modulation and coupling to the input ring 44 whichmay employ a 2-D spatial light modulator 91 coupled to the data from theprocessing elements 25. Free space to fiber optical system 92 is coupledto the 2-D spatial light modulator and the input ring 44.

A complex component in the system is the one containing the datamodulators, i.e. the optical-switching devices. Although differentswitching devices may be used, waveguide switching devices are wellsuitable for this purpose. Waveguide switches have been investigated andused in optical integrated circuits for almost twenty years. See L.Thylen IEEE J. Lightwave Technol., vol. 6, p. 847, 1988. The most commontype of waveguide switches is the interferometric Mach-Zehnder (M-Z)switch shown in FIG. 6. Light is coupled into a single-node fiber, andthen split evenly into two paths forming a Y junction.

The light from the two paths is then combined into a single modewaveguide 67. Depending on the relative phase delay between the twooptical paths, the recombined light output can be switched from amaximum to a minimum power. This phase delay can be controlled by anelectronically applied voltage. M-Z devices can operate at anywavelength at which the waveguide is transparent, and thus areappropriate for use in the system architecture. M-Z devices can be alsobe used as a data modulator. The maximum data rates at which the systemcan operate is not limited by the bandwidth of M-Z devices. Currently,LiNbO₃ based semiconductor waveguide M-Z switches have been operated at40 GHz while maintaining greater than 20-dB on/off extinction ratio.Electro-optic polymers may also be candidates for fast speed switchingdevices because of their compatibility with optoelectronic integration.

The holographic-optical elements are key components in this interconnectsystem. The requirements of the holographic-optical elements are highdiffraction efficiency, high spectral selectivity, and low absorption.The holographic-optical elements shown in FIG. 4 are reflection typeholograms, though both reflection and transmission type holograms may beused.

Because of their higher wavelength selectivity, reflection holograms aredesirable. To estimate the wavelength selectivity of a reflectionhologram, a brief analysis is given. According to Kogelnik H. Kogelnik,“Coupled wave theory for thick hologram gratings,” Bell Syst. Tech. J.,vol 48, pp. 2909-2947, 1969, the amplitude of the diffracted wave for anunslanted reflection grating is expressed by $\begin{matrix}{{S = \frac{- i}{{\left( {{\frac{i}{\xi}\quad \gamma_{r}} +} \right.1} - {\left( {\xi \quad \gamma_{r}^{1/2}} \right.^{1/2}\quad \coth \quad \left( {\gamma_{r}^{2} - \xi_{r}^{2}} \right)^{1/2}}}},\quad {where}} & (1) \\{{\upsilon_{r} = {\pi \quad n_{1}\quad \frac{T}{\lambda \quad \sin \quad \theta}}},} & (2) \\{{\xi = {{\delta \quad \beta \quad T\quad \cos \quad \theta} = {\frac{2\quad \pi}{\lambda}\quad n_{0}\quad \delta \quad T\quad \cos \quad \theta}}},} & (3)\end{matrix}$

δ is the deviation of the incident angle away from the Bragg angle θ₀given by δ=θ−θ₀. T is the thickness of the recording material, λ is theillumination wavelength, n₁ is the index modulation, n₀ is the index ofrefraction of the material, and .theta. is the incident angle. The Braggcondition is expressed by

2d n ₀ sin θ₀=λ₀,  (4)

where d is the grating spacing and λ₀ is the wavelength satisfying theBragg condition. Under Bragg condition, maximum diffraction efficiencyis obtained. Letting the wavelength of the illumination be changed toλ₀+δλ₀, the maximum diffraction efficiency is no longer obtained at theillumination angle θ₀, but instead is given by the new Bragg angleθ′₀=θ₀+δθ. If we maintain the illumination at the original angleθ₀=θ′₀−δθ, diffraction efficiency is reduced according the deviation.delta..theta. from the Bragg angle θ′₀. The spectral selectivity isdefined as the width δλ until the diffraction efficiency goes to zero.The Bragg condition given in Equation 4 may be expressed in terms of δλas

2d n ₀ sin(θ₀+δθ)=λ₀+δλ,  (5)

Making some approximations and using Equation 4, yields $\begin{matrix}{{\delta \quad \theta_{0}} = {{- \frac{\delta \quad \lambda}{\lambda}}\quad \tan \quad {\theta_{0}.}}} & (6)\end{matrix}$

Substituting the above equations into Equation 1, the formula for thediffraction efficiency is obtained. FIG. 10 illustrates the diffractionefficiency dB vs. δλ. The parameters used were λ=1.3 μm, T=500 μm,n=1.52, θ=π/4, and υ=0.95π. The curve shows that for these parameters,the spectral selectivity is equal to 1.2 nm. When holographic-opticalelements are used, crosstalk is induced by light from the zeroth order.In the above case, the crosstalk is less than −20 dB, which isequivalent to a contrast ratio of 100:1 in an intensity switchingconfiguration. Evidently, the high wavelength selectivity and the lowcrosstalk will allow us to employ a large number of channels. Thisarchitecture, while maintaining all the advantages of past ringtopologies, overcomes current switching device problems such as slowspeed and lack of appropriate 2-D array devices. Employing waveguideswitches, the system is capable to operate at data rates up to 40 GHz.The use of holographic-optical elements makes possible to self-route thewavelength coded input data. With the development of waveguide switches,the proposed architecture leads to a very powerful interconnect systemfor the implementation of SIMD machines.

To ensure the full functionality of the ring topology based optical freespace n interconnect system, device issues should be addressed. Twobasic devices are needed in the implementation of this architecture:optical transmitters/receivers and optical switching devices.Theoretically, since the system has identical latency, the interconnectsystem does not impose any limitation on the system data rate(s).

The present invention also includes a method for opticallyinterconnecting a plurality of SIMD processors. The method includes thesteps of generating a control signal from a processor, and generating aplurality of coherent-light beams at a plurality of wavelengths,respectively. In response to the control signal, the method selects afirst coherent-light beam at a first wavelength from a plurality ofwavelengths, and combines and divides the output light for each of theplurality of wavelengths into a plurality of equal-length paths. Each ofthe equal-length paths corresponds to each of the SIMD processors. Usingdata from a respective SIMD processor, the method modulates,corresponding to the light with selected wavelength, the output light,and forms with the output light at a respective wavelength, and inputring at the optical interconnect.

The method includes using the optical interconnect for reflecting fromthe input plane the output light, and in response to the control signal,selecting an optical channel for the reflected output light. The lightfrom the selected optical channel is reflected to an output-ringdetector array, and is thereby detected by the detectors. The detectedsignal is fed back to the SIMD processors.

The method performs the above steps for each wavelength, and thenrepeats the entire sequence for all wavelengths.

It will be apparent to those skilled in the art that variousmodifications can be made to the optical interconnect for high speedprocessors of the instant invention without departing from the scope orspirit of the invention, and it is intended that the present inventioncover modifications and variations of the optical interconnect for highspeed processors provided they come within the scope of the appendedclaims and their equivalents.

What is claimed is:
 1. A method for transferring data between aplurality of processors, the method comprising: producing a plurality oflight beams, the light beams having at a particular time a singlewavelength, the single wavelength selected from a plurality ofwavelengths; each light beam uniquely associated with a processor; eachprocessor selectively modulating data onto its light beam; controllingat a particular time the selected wavelength of the single wavelengthand the processors modulating data onto their light beam; receiving theplurality of light beams at an input ring array; transferring the lightbeams from the input ring array to an output ring array and rotatingeach light beam, each selected wavelength uniquely associated with aparticular rotation; and outputting a light beam to each processor fromthe output ring array.
 2. The method of claim 1 further comprising:producing the plurality of wave lengths; and selectively combining thewavelengths and splitting the combined wavelength to produce theplurality of light beams.
 3. The method of claim 2 wherein theselectively combining is performed by a plurality of switches coupled toa combiner.
 4. The method of claim 1 wherein the producing the pluralityof light beams is performed by a tunable laser.
 5. An apparatuscomprising: means for producing a plurality of light beams, the lightbeams having at a particular time a single wavelength, the singlewavelength selected from a plurality of wavelengths, the singlewavelength selected from a plurality of wavelengths, and each light beamuniquely associated with a processor of a plurality of processors; theplurality of processors, each processor selectively modulating data ontoits light beam; means for controlling at a particular time the selectedwavelength of the single wavelength and the processors modulating dataonto their lightbeams; an input ring receiving the plurality of lightbeams at an input ring array; means for transferring the light beamsfrom the input ring array to an output ring array and rotating eachlight beam, each selected wavelength uniquely associated with aparticular rotations; and the output ring array for outputting a lightbeam to each processor.
 6. The apparatus of claim 5 wherein theproducing the plurality of light beams means comprises: means forproducing the plurality of wavelengths; and means for selectivelycombining the wavelengths and splitting the combined wavelengths toproduce the plurality of lightbeams.
 7. The apparatus of claim 5 whereinthe selectively combining is performed by a plurality of switchescoupled to a combiner.
 8. The apparatus of claim 5 wherein the producingthe plurality of light beams means comprises a tunable laser.